White luminescent phosphor for use in cathode ray tube

ABSTRACT

Disclosed is a white luminescent phosphor for use in a cathode ray tube, which comprises a mixture of at least one phosphor having a composition of the following formula: 
     
         In.sub.1-p-q-r M.sub.p Eu.sub.q Tb.sub.r BO.sub.3 
    
     wherein M represents at least one element selected from the group consisting of Sc, Lu, Y, La, Gd and Ga; and each of p, q and r represents a number satisfying the relationships of p≧0, q≧0, r≧0 and 0&lt;p+q+r&lt;1, and having a 10% afterglow period of 10 milliseconds or longer, and a blue luminescent phosphor having the composition of ZnS:Ag.

BACKGROUND OF THE INVENTION

This invention relates to an improved white luminescent mixed phosphorfor use in a cathode ray tube.

For preparation of a white luminescent fluorescent screen of a cathoderay tube for terminal display such as a display tube, because of theabsence of a practically available single white luminescent fluorescentphosphor, it has been practiced in the art to employ a mixture ofseveral phosphors with different luminescent colors at appropriateproportions.

Examples of such white luminescent mixed phosphors known in the art mayinclude a mixture of a blue-emitting silver-activated zinc sulfidephosphor (Zns:Ag) and a copper-activated zinc-cadmium sulfide phosphor{(Zn,Cd)S:Cu} emitting light with a complementary color thereof mixed ata suitable proportion so as to emit substantially white color byelectron beam excitation (disclosed in U.S. Pat. No. 3,891,886), or amixture of the three kinds of phosphors of the above blue-emittingZnS:Ag, a green-emitting manganese-arsenic-activated zinc silicatephosphor (Zn₂ SiO₄ :Mn,As) and a red-emitting manganese-activated zincphosphate phosphor {Zn₃ (PO₄)₂ :Mn} or manganese activatedzinc-magnesium phosphate phosphor {(Zn,Mg)₃ (PO₄)₂ :Mn} mixed at asuitable proportion so as to emit substantially white color by electronbeam excitation (disclosed in National Technical Report 25, 251 1978).

However, in the case of the former mixed phosphor, since its 10%afterglow period is less than 10 milliseconds (ms), the cathode-ray tubehaving its fluorescent screen constituted of this mixed phosphor cannotbe free from the drawback of greater flicker on its picture screen.

In the case of the latter mixed phosphor, while the flicker on thefluorescent screen can be alleviated due to the longer afterglow periodof each of Zn₂ SiO₄ :Mn,As and Zn₃ (PO₄)₂ :Mn or (Zn,Mg)₃ (PO₄)₂ :Mn onone hand, there is involved the drawback that no sufficient luminancecan be obtained, on the other hand.

For solving such a problem, the present inventors have proposed a mixedphosphor employing, in place of the red-emitting component in the lattermixed phosphor, an orange-emitting phosphor, for example, a manganeseactivated cadmium chlorophosphate {Cd₅ Cl(PO₄)₃ :Mn} phosphor. Thismixed phosphor has sufficient luminance and is also excellent in flickeralleviating effect.

However, in this mixed phosphor, since a harmful element of cadmium iscontained in Cd₅ Cl(PO₄)₃ :Mn as shown above as an exemplaryorange-emitting phosphor, it cannot be stated to be preferable from theviewpoint of preventing pollution. Also, in the case of this mixedphosphor, the matrices for the respective component phosphors to bemixed are entirely different materials, and therefore it is difficult toobtain a homogeneous mixture by mixing these componente. Accordingly, afluorescent screen constituted of this mixed phosphor had the drawbackthat irregularity is liable to be formed in the color emitted.

On the other hand, there is known a phosphor principally comprised ofInBaO₃ and activated with any one of Tb, Eu, Dy and Sm (disclosed inU.S. Pat. No. 3,394,084). However, what is intended in this prior art bythe incorporation of these rare earth elements is merely to obtainphosphors of different cathodoluminescence color by incorporation of anyone of Tb, Eu, Dy and Sm; for instance, green cathodoluminescence byTb-activated InBaO₃, or red cathodoluminescence by Eu-activated InBaO₃.

SUMMARY OF THE INVENTION

An object of this invention is to provide a white luminescent mixedphosphor for use in a cathode ray tube, having sufficient luminanceunder electron beam excitation and the effect of alleviating flicker,containing no harmful element of cadmium and also capable ofconstituting a fluorescent screen with little color irregularity.

The present inventors have found the facts that, in the CIE chromaticitydiagram shown in the drawing, the europium activated indium borate(InBO₃ :Eu) emits light with high efficiency to the reddish orangeindicated by the point G (x=0.604, y=0.396 in the CIE chromaticitycoordinate), while the terbium activated indium borate (InBO₃ :Tb) emitslight with high efficiency to the green indicated by the point F(x=0.325, y=0.628 in the CIE chromaticity coordinate), and both having a10% afterglow period of 10 ms or longer; and further that the indiumborate activated with both of Eu and Tb emits light with high efficiencyhaving a chromaticity on the straight line connecting between the abovepoints G and F by suitable selection of its activation amount, its 10%afterglow period being also 10 ms or longer. As a consequence, thepresent inventors have thought of an idea that a mixed phosphor emittingyellow light complementary to blue color can be obtained by mixing thesephosphors at a suitable proportion, and have further made studies tosuccessfully develop the white luminescent mixed phosphor of thisinvention.

More specifically, the white luminescent phosphor of this inventioncomprises a mixture of at least one phosphor having a composition of thefollowing formula:

    In.sub.1-p-q-r M.sub.p Eu.sub.q Tb.sub.r BO.sub.3

wherein M represents at least one element selected from the groupconsisting of Sc, Lu, Y, La, Gd and Ga; and each of p, q and rrepresents a number satisfying the relationships of p≧0, q≧0, r≧0 and0<p+q+r<1, and also having a 10% afterglow period of 10 milliseconds orlonger, and a blue luminescent phosphor having the composition ofZnS:Ag.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a chromaticity diagram showing the scope of the JEDEC standardrepresented by the parallerogram ABCD, the white color of 6500 K.+7MPCDrepresented by the point E, the luminescent color of InBO₃ :Tbrepresented by the point F, the luminescent color of InBO₃ :Eurepresented by the point G and the luminescent color of ZnS:Agrepresented by the point H.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The "white" as herein mentioned refers to the emitted color having thechromaticity within the standard of JEDEC (Joint Electron DeviceEngineering Councils) surrounded by the four points of the point A(x=0.273, y=0.282), the point B (x=0.267, y=0.303), the point C(x=0.286, y=0.326) and the point D (x=0.290, y=0.303), the vicinitythereof, the point indicated by the color temperature of 6500 K.+7MPCDand the vicinity thereof.

In this invention, the phosphor shown by the compositional formla ofIn_(1-p-q-r) M_(p) Eu_(q) Tb_(r) BO₃ is an orange- to yellow-emittingphosphor.

This is a phosphor comprising InBO₃ having a calcite crystallinestructure as the matrix, and p, q, r are indices indicating mols of therespective elements. Each of p, q and r is a decimal of 0 or greater,but these cannot be zero at the same time. These p, q and r are selectedappropriately depending on such factors as the chromaticity of thedesired emitted color of the phosphor, the emission efficiency, theinfluence on the crystalline structure of the matrix, etc. and thereforethey cannot be determined unequivocally. Generally, p is 0.2 or less, qis 0.04 or less and r is 0.04 or less.

Accordingly, this phosphor is at least one of or a mixture of two ormore of a InBO₃ :Eu,Tb alone wherein p=0 and q, r are appropriatenumbers; a mixture of InBO₃ :Eu wherein p=r=0 and q is an appropriatenumber, InBO₃ :Tb wherein p=q=0 and r is an appropriate number and InBO₃:Eu,Tb wherein p=0 and q, r are appropriate numbers mixed in a suitablecombination; and each of the above cases, wherein a part of In issubstituted in p mols (p=0) with at least one element selected from thegroup consisting of ScLu, Y, Gd and Ga. And, these phosphors in any caseare required to have a 10% afterglow period of 10 ms or longer. If it isless than 10 ms, no flicker alleviating effect can be exhibited.

These phosphors can be prepared readily according to a conventionalprocedure by mixing the powders of the element sources such as oxides ofthe respective elements in appropriate amounts to a desired composition,calcining at a certain temperature the powder mixture obtained in avessel in which no impurity is entrained, and then subjecting thecalcinined product to cooling, washing with water, filtration, dryingand screening.

The other component in the white luminescent mixed phosphor of thisinvention is a blue luminescent phosphor having a 10% afterglow periodof 10 μs or longer. With a 10% afterglow period shorther than 10 μs, theflicker alleviating effect is undesirably lowered. As an example of sucha blue luminescent phosphor, there may be employed the above-mentionedZnS:Ag.

The mixed phosphor of this invention can be prepared easily by mixingthe respective phosphors at desired proportions. The proportionsemployed cannot be determined unequivocally, since they are differentdepending on the chromaticity of the respective phosphors employed inthe CIE chromaticity diagram and their emission efficiencies. Generallyspeaking, the blue-emitting phosphor may be used in an amount within therange of from 10 to 40%, preferably 10 to 30%, in terms of weight ratio.

The thus prepared white luminescent mixed phosphor can be used for acathode ray tube by applying it on the display screen of a cathode-raytube by a conventional coating method to provide a luminescent layerthereon.

EXAMPLES 1 TO 10

The respective phosphors of the compositional formulae as shown in Table1 were mixed at proportions indicated in the Table. Also, cathode-raytubes having the fluorescent screens constituted of the respective mixedphosphors were prepared and the color irregularities under conventionalelectron beam excitation were observed with naked eyes. Color uniformity(herein meant to be chromaticity difference between central area andperipheral area of the fluorescent screen; √(Δx)² +(Δy)²) was alsomeasured. Results of these are summarized in Tables 1a.

The luminance is represented in terms of the relative luminance to theluminance of a commercially available Zn₂ SiO₄ :Mn,As phosphor (JEDECregistered P39 phosphor) as 100%, and the critical fusion frequency(herein meant to be the number per second of displayed images in whichflickers start to appear) is a measured value at the luster luminance of20 ft-L.

In the Tables, Table 1 shows the composition and the CIE chromaticitycoordinate of phosphors of Examples 1 to 10 and Comparative Examples 1to 5; and Table 1a, the luminance, the critical fusion frequency, theharmfulness, the color irregularity and the color uniformity of thecorresponding Examples and Comparative examples.

                  TABLE 1                                                         ______________________________________                                               *(P22 blue component)                                                         Compositional formula of                                                                        CIE chroma-                                                 phosphor and proportions                                                                        ticity                                                      thereof (wt. %)   coordinate                                           ______________________________________                                        Example 1:                                                                             ZnS:Ag*          25     x = 0.276                                             In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                             47.5   y = 0.302                                             In.sub.0.97 Eu.sub.0.03 BO.sub.3                                                               27.5                                                Example 2:                                                                             ZnS:Ag*          20     x = 0.276                                             In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                             42     y = 0.302                                             In.sub.0.996 Eu.sub.0.003 Tb.sub.0.001 BO.sub.3                                                38                                                  Example 3:                                                                             ZnS:Ag*          25     x = 0.276                                             In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                             47.5   y = 0.302                                             In.sub.0.76 Y.sub.0.2 Eu.sub.0.04 BO.sub.3                                                     27.5                                                Example 4:                                                                             ZnS:Ag*          25     x = 0.276                                             In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                             47.5   y = 0.302                                             In.sub.0.86 Sc.sub.0.1 Eu.sub.0.04 BO.sub.3                                                    27.5                                                Example 5:                                                                             ZnS:Ag*          26     x = 0.276                                             In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                             43     y = 0.302                                             In.sub.0.92 Lu.sub.0.05 Eu.sub.0.03 BO.sub.3                                                   31                                                  Example 6:                                                                             ZnS:Ag*          25     x = 0.276                                             In.sub.0.945 La.sub.0.05 Tb.sub.0.005 BO.sub.3                                                 49     y = 0.302                                             In.sub.0.97 Eu.sub.0.03 BO.sub.3                                                               26                                                  Example 7:                                                                             ZnS:Ag*          26     x = 0.276                                             In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                             45     y = 0.302                                             In.sub.0.96 Gd.sub.0.01 Eu.sub.0.03 BO.sub.3                                                   29                                                  Example 8:                                                                             ZnS:Ag*          27     x = 0.276                                             In.sub.0.985 Ga.sub.0.01 Tb.sub.0.005 BO.sub.3                                                 44     y = 0.302                                             In.sub.0.97 Eu.sub.0.03 BO.sub.3                                                               29                                                  Example 9:                                                                             ZnS:Ag*          26     x = 0.276                                             In.sub.0.9974 Eu.sub.0.0007 Tb.sub.0.0019 BO.sub.3                                             74     y = 0.302                                    Example 10:                                                                            ZnS:Ag*          26     x = 0.276                                             In.sub.0.9974 Eu.sub.0.0007 Tb.sub.0.0019 BO.sub.3                                             59     y = 0.302                                             In.sub.0.9945 Eu.sub.0.0005 Tb.sub.0.005 BO.sub.3                                              11                                                           In.sub.0.996 Eu.sub.0.0035 Tb.sub.0.0005 BO.sub.3                                              4                                                   Comparative                                                                            ZnS:Ag*          45     x = 0.276                                    Example 1:                                                                             (Zn,Cd)S:Cu      55     y = 0.302                                    Comparative ZnS:Ag*                                                                    17               x = 0.276                                           Example 2:                                                                             Zn.sub.2 SiO.sub.4 :Mn,As                                                                      30     y = 0.302                                             Zn.sub.3 (PO.sub.4).sub.2 :Mn                                                                  53                                                  Comparative                                                                            ZnS:Ag*          19     x = 0.276                                    Example 3:                                                                             Zn.sub.2 SiO.sub.4 :Mn,As                                                                      19     y = 0.302                                             Cd.sub.5 Cl(PO.sub.4).sub.3 :Mn                                                                62                                                  Comparative                                                                            ZnS:Ag*          22     x = 0.276                                    Example 4:                                                                             Zn.sub.2 SiO.sub.4 :Mn,As                                                                      38     y = 0.302                                             In.sub.0.97 Eu.sub.0.03 BO.sub.3                                                               40                                                  Comparative                                                                            ZnS:Ag*          21     x = 0.276                                    Example 5:                                                                             In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                             39     y = 0.302                                             Zn(PO.sub.4).sub.2 :Mn                                                                         40                                                  ______________________________________                                    

                  TABLE 1a                                                        ______________________________________                                                                            Color uniformity                          Exam- Lumi-   Critical        Color (Center/corner                            ple   nance   fusion    Harm- irregu-                                                                             chromaticity                              No.   (%)     frequency fulness                                                                             larity                                                                              difference)                               ______________________________________                                        1     75      56        none  none  0.005                                     2     74      56        none  none  0.005                                     3     75      56        none  none  0.005                                     4     75      56        none  none  0.005                                     5     72      56        none  none  0.005                                     6     69      56        none  none  0.005                                     7     74      56        none  none  0.005                                     8     76      56        none  none  0.005                                     9     73      56        none  none  0.005                                     10    73      56        none  none  0.005                                     Comparative Example:                                                          1     103     58        do    none  0.005                                     2     50      53        none  do    0.007                                     3     77      53        do    do    0.01                                      4     64      54        none  do    0.007                                     5     60      55        none  do    0.007                                     ______________________________________                                    

As can be seen from the foregoing, the mixed phosphor of this inventionhas sufficiently great luminance and also low in critical fusionfrequency, thus indicating flicker alleviating effect is, harmless dueto the absence of cadmium, and is also excellent in homogeneity duringmixing because of the use of the same or the same kind of a phosphormatrix for formation of yellow which is complementary to blue. As aresult, irregularity of the emitted color on the fluorescent screen canbe reduced, and its commercial value is great.

We claim:
 1. A white emitting luminescent phosphor for use in a cathoderay tube, which consists essentially of a mixture of from about 10 to 40wt.% of a blue luminescent phosphor having the composition of ZnS:Ag,and having a 10% afterglow period of at least 10 microseconds, and atleast one phosphor having a composition of the following formula:

    In.sub.1-p-q-r M.sub.p Eu.sub.q Tb.sub.r BO.sub.3

wherein M represents at least one element selected from the groupconsisting of Sc, Lu, Y, La, Gd and Ga; and each of p, q, and rrepresents a number satisfying the relationships of p≧0, q≧0, r≧0 and0<p+q+r<1, and having a 10% afterglow period of 10 milliseconds orlonger.
 2. The phosphor according to claim 1, wherein said numbersrepresented by p, q and r are 0.2≧p≧0, 0.04≧q≧0, and 0.04≧r≧0,repsectively.
 3. The phosphor according to claim 1, wherein said ZnS:Agphosphor is mixed in an amount ranging from 10 to 30 wt.%.
 4. Thephosphor according to claim 1, wherein said white emitting luminescentphosphor has the following composition:

    ______________________________________                                        ZnS:Ag                25 wt. %                                                In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                                  47.5 wt. %                                              In.sub.0.97 Eu.sub.0.03 BO.sub.3                                                                    27.5 wt. %.                                             ______________________________________                                    


5. The phosphor according to claim 1, wherein said white emittingluminescent phosphor has the following composition:

    ______________________________________                                        ZnS:Ag                 20 wt. %                                               In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                                   42 wt. %                                               In.sub.0.996 Eu.sub.0.003 Tb.sub.0.001 BO.sub.3                                                      38 wt. %.                                              ______________________________________                                    


6. The phosphor according to claim 1, wherein said white emittingluminescent phosphor has the following composition:

    ______________________________________                                        ZnS:Ag                25 wt. %                                                In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                                  47.5 wt. %                                              In.sub.0.76 Y.sub.0.2 Eu.sub.0.04 BO.sub.3                                                          27.5 wt. %.                                             ______________________________________                                    


7. The phosphor according to claim 1, wherein said white emittingluminescent phosphor has the following composition:

    ______________________________________                                        ZnS:Ag                25 wt. %                                                In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                                  47.5 wt. %                                              In.sub.0.86 Sc.sub.0.1 Eu.sub.0.04 BO.sub.3                                                         27.5 wt. %.                                             ______________________________________                                    


8. The phosphor according to claim 1, wherein said white emittingluminescent phosphor has the following composition:

    ______________________________________                                        ZnS:Ag                 26 wt. %                                               In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                                   43 wt. %                                               In.sub.0.92 Lu.sub.0.05 Eu.sub.0.03 BO.sub.3                                                         31 wt. %.                                              ______________________________________                                    


9. The phosphor according to claim 1, wherein said white emittingluminescent phosphor has the following composition:

    ______________________________________                                        ZnS:Ag                 25 wt. %                                               In.sub.0.945 La.sub.0.05 Tb.sub.0.005 BO.sub.3                                                       49 wt. %                                               In.sub.0.97 Eu.sub.0.03 BO.sub.3                                                                     26 wt. %.                                              ______________________________________                                    


10. The phosphor according to claim 1, wherein said white emittingluminescent phosphor has the following composition:

    ______________________________________                                        ZnS:Ag                 26 wt. %                                               In.sub.0.995 Tb.sub.0.005 BO.sub.3                                                                   45 wt. %                                               In.sub.0.96 Gd.sub.0.01 Eu.sub.0.03 BO.sub.3                                                         29 wt. %.                                              ______________________________________                                    


11. The phosphor according to claim 1, wherein said white emittingluminescent phosphor has the following composition:

    ______________________________________                                        ZnS:Ag                 27 wt. %                                               In.sub.0.985 Ga.sub.0.01 Tb.sub.0.005 BO.sub.3                                                       44 wt. %                                               In.sub.0.97 Eu.sub.0.03 BO.sub.3                                                                     29 wt. %.                                              ______________________________________                                    


12. The phosphor according to claim 1, wherein said white emittingluminescent phosphor has the following composition:

    ______________________________________                                        ZnS:Ag                  26 wt. %                                              In.sub.0.9974 Eu.sub.0.0007 Tb.sub.0.0019 BO.sub.3                                                    74 wt. %.                                             ______________________________________                                    


13. The phosphor according to claim 1, wherein said white emittingluminescent phosphor has the following composition:

    ______________________________________                                        ZnS:Ag                  26 wt. %                                              In.sub.0.9974 Eu.sub.0.0007 Tb.sub.0.0019 BO.sub.3                                                    59 wt. %                                              In.sub.0.9945 Eu.sub. 0.0005 Tb.sub.0.005 BO.sub.3                                                    11 wt. %                                              In.sub.0.996 Eu.sub.0.0035 Tb.sub.0.0005 BO.sub.3                                                      4 wt. %.                                             ______________________________________                                    